Is quitting my job due to mental health a good enough excuse?

Your mental health is always a suitable explanation of withdrawing from anything, especially your place of employment. We have to take care of ourselves and taking care of our mental health is no different than addressing a physical ailment. It's imperative that you are assessing if your job is adding to or taking away from your life. If you are dreading going into work, scared, unmotivated, detached (floating through), and in tears it seems highly likely that a change in employment is necessary, if you are unable to resolve the underlying causation to your current way of feeling. 

To leave a job is a big decision, as it's your livlihood. I would encourage you to assess what you have control over in this situation and to optimize those aspects. If you have not shared with your direct supervisor, or higher ups of your concerns then I'd encourage you to do so. It may be that you need to take a pause in your employment, which can be something that you, your employer, and perhaps your physican could discuss as there are different leave of absences available to most individuals. 

I do hear that whatever has taken place at work has been a triggering event. There seems to be a level of new trauma or a triggering event to a past trauma. I don't know the entirey of the details but it seems that it's important for you to become acknowledge how you are feeling, placing a name to the emotion, and to then explore the underlying root of causation. Therapy is a great tool to pull back the layers to ourselves to explain why we respond and react to life, the way that we do. 

I hope that you are able to communicate with someone that can assist you directly, to then make a plan of action of your continued employment with this employer. It's important that we let others know how we are feeling, because if the people that need to know don't know how we are feeling, then they cannot do anything to help us in our problems. Please reach out and take care of your mental health, as it's just as important as anything else in life.